1. Field of the Invention
The present invention relates to microelectronic memories, and in particular to a circuit for discharging the cells of such a memory without using the word lines.
2. Description of the Prior Art
Since their relatively recent development, semiconductor memories have increased dramatically in size. Semiconductor memories which store more than 4,000 bits (4K) or 8,000 bits (8K) are now widely commercially available. Arrays of this size are small enough that the physical limits for materials from which they are constructed generally are not approached. A discussion of microelectric memories is provided in Scientific American, September, 1977, pages 135 through 139.
The recent introduction of 16K, 64K and larger microelectronic memories has created several problems. Among these problems are the phenomena of electromigration or metal migration along conductors and memory cell current saturation.
In most 16K or larger memories, the memory cells are arranged in rows (word lines) and columns (bit lines). Each row has a word line conductor to which each memory cell for that row is connected. Because of the large number of memory cells on each word line in such memories, the word line conductors are relatively long. To increase the packing density of the memory array and all of its associated circuitry on a chip of silicon, the thickness of all conductors, including the word line conductors, must be minimized.
Metal migration occurs when a metal conductor, such as the word line conductor in a microelectronic memory has a high ratio of current flow to cross-sectional area. In large memory arrays, a relatively large amount of current is needed to operate all of the cells on the line. Because the word line conductor may have a cross-sectional area of only a few square microns, current flowing along the word line conductor of only a milliamp may cause metal migration. Metal migration along the conductors results in degradation and eventual disintegration of the conductor.
The problem of memory cell current saturation presents another difficulty in developing the technology of 16K and larger microelectronic memories. The presence of increased current on a selected word line saturates the individual memory cells, degrading their response time significantly. It is not unusual for memory cells to respond ten times more slowly when saturated than in the absence of cell current saturation.
In large microelectronic memory arrays, metal migration and cell current saturation are often exacerabated by certain business practices. A computer system incorporating microelectronic memories occasionally may be turned off. In normal operation, the word lines in the memory are scanned very quickly such that the large currents are present for only a fraction of a second. When the machine is turned off, the word lines are no longer scanned, and a particular word line may remain energized for several hours. It is possible to cause significant damage to the microelectronic memory by allowing such a large flow of current over an extended period of time on a single word line.